The present invention relates to a radiation imaging apparatus such as a radiation video camera system for converting a radiation projection image of an object to a visible image and detecting the visible image with a video camera. The present invention relates to a radiation imaging method using such an apparatus. The present invention relates to, for example, a radiation image diagnosis apparatus for medical use.
According to a conventional radiation imaging apparatus, particularly an X-ray video camera system, a camera tube and solid-state image sensor which are generally used for broadcasting are used and an X-ray image intensifier or fluorescent screen is used as a means to convert X rays to a visible image. As a high sensitivity X-ray video camera system, there is an X-ray video camera system using a secondary electron multiplying type camera tube such as a silicon intensifier target (SIT) tube as a high sensitivity camera tube. As a high sensitivity device, there is a solid-state image sensor with a visible light image intensifier. According to a conventional normal video camera, light irradiated to the camera tube is converted to electric charges in the number in proportion to the number of light photons in the photoelectric conversion layer and the electric charges in the number in proportion to the number of irradiated light photons reach the electrode and are converted to a signal current. Therefore, when an image to be detected has a wide-gray scale level distribution, the detected image also has a wide gray scale level distribution. As a result, when the entire detected image is displayed after imaging, the contrast of the specific region of interest is low. To obtain an X-ray image in which the gray scale level of the background is uniform as much as possible, an X-ray attenuation filter in a suitable shape is used conventionally between an object and the X-ray source. The portion having high X-ray transmissivity in the object has a high gray scale level in the detected image and the portion having low X-ray transmissivity has a low gray scale level in the detected image. To correct it, the X-ray attenuation filter at the position corresponding to the portion having high X-ray transmissivity in the object is made thick so as to absorb a large amount of X-rays and to reduce the X-ray radiation exposure of the object. And, the X-ray attenuation filter at the position corresponding to the portion having low X-ray transmissivity in the object is made thin so as to absorb a small amount of X rays and to increase the X-ray radiation exposure of the object. By using such X-ray attenuation filters, the X-ray transmissivity distribution in each region of the object is corrected and an X-ray image in which the gray scale level of the background is uniform as much as possible is obtained as a detected image.
In addition to the above, as a method for detecting an image in which the gray scale level of the background is almost uniform, there is a method for changing the amplification factor of the amplifier circuit for video camera output signals in correspondence with the electron beam scanning position of the camera tube according to a synchronizing signal of the video camera. Namely, when reading out the portion of an input image with high signal intensity by electron beam scanning of the camera tube, the amplification factor of the amplifier circuit for video camera output signals is decreased. When reading out the portion of an input image with low signal intensity, the amplification factor of the amplifier circuit is increased. By detecting an image like this by changing the amplification factor of the amplifier circuit at each electron beam scanning position of the camera tube, an image is detected by converting the gray scale level distribution of the background of the detected image to an almost uniform gray scale level distribution.
Apparatuses related to this type of apparatus are indicated in, for example, Japanese Patent Application Laid-Open Nos. 64-17631, 1-300684, 1-91837 and 2-41142, and U.S. Pat. No. 5,023,896 which corresponds to Japanese Patent Application Laid-Open No. 1-300684.
In the conventional camera tube and solid-state image sensor which are used generally for broadcasting, the sensitivity of the imaging device itself is low. In the X-ray image intensifier or fluorescent screen, the brightness of an output visible image in imaging at a low X-ray dose is low. In an X-ray video camera system using a secondary electron multiplying type camera tube such as an SIT tube as a high sensitivity camera tube, the resolution is low. In a solid-state image sensor with a visible light image intensifier, the resolution is also low.
In addition to the above problems, in the conventional normal video camera, when an image to be detected has a wide gray scale level distribution, the detected image also has a wide gray scale level distribution and as a result, when the entire detected image is displayed after imaging, the contrast of the specific region of interest is low. When the displaying brightness level of the display device is adjusted so that only the region of interest is enhanced so as to make it easy to see, the gray scale level distribution of the entire image is out of the displayable range of the display device and the image cannot be displayed at a suitable brightness level. According to the conventional method using an X-ray attenuation filter in a suitable shape between an object and the X-ray source so as to solve the above problem and to obtain an X-ray image which is uniform as much as possible, it is necessary to mount an X-ray. attenuation filter for each object and the throughput is not high. Furthermore, according to the conventional method for changing the amplification factor of the amplifier circuit for video camera output signals at each electron beam scanning position of the camera tube. according to a synchronizing signal of the video camera, the amplification factor of the amplifier circuit is changed in correspondence with the electron beam scanning position of the camera tube.
Therefore, the image signal intensity is changed, and the circuit noise component is simultaneously amplified in proportion to the amplification of the signal component, and a noise distribution is generated in the detected image.